As is known, lithography is commonly used when manufacturing integrated circuits. The process generally includes coating a photoresist layer on the surface of a semiconductor wafer, and then shining light through a mask with the desired pattern and focusing the light into the light sensitive photoresist. The mask typically has light non-transmissive (opaque) regions of chrome and light transmissive (transparent) regions of quartz. Radiation from a light source (e.g., ultra-violet or deep ultra-violet light, etc) and focused via an optical lens system is then applied to the mask. In any such eases, the light passes through the transparent mask regions and exposes the corresponding portion of the photoresist layer, and is blocked by the opaque mask regions to leave the other portions of photoresist layer unexposed. Depending on the specific process used, either the exposed or non-exposed regions of photoresist layer can then be removed, thereby leaving a patterned resist layer on the wafer, which in turn allows for subsequent processing of the wafer such as, for example, etching, depositing, and other typical semiconductor processes.
One conventional technique that enables smaller minimum device dimensions with relatively good contrast is generally referred to as alternating phase shift masking (APSM). APSM exploits the destructive interference caused by light passing through two adjacent transparent mask areas to create an unexposed area on the photoresist layer. In short, the phase of the light wave passing through and exiting the transparent mask regions is a function of the mask thickness, so mask thickness of adjacent mask regions can be selectively varied, such that the light exiting from those adjacent regions is out-of-phase which causes destructive interference (e.g., were light waves exiting from one adjacent area have a phase that is out-of-phase with the phase of light waves exiting from the other adjacent area, and the waves therefore cancel one another or otherwise yield a combined wave of diminished amplitude). Since the photoresist material is responsive to the intensity of the light, an unexposed area will be formed on the photoresist layer where the transparent regions of differing thicknesses are adjacent. Other mask technologies, such as chromeless phase shift lithography (CPL), can also be used to introduce phase offset between light passing through some parts of the mask relative to other parts of the mask.